Tutorial: Simple WEP Crack

Version: 1.20 January 11, 2010
By: darkAudax

Introduction

This tutorial walks you though a very simple case to crack a WEP key. It is intended to build your basic skills and get you familiar with the concepts. It assumes you have a working wireless card with drivers already patched for injection.

The basic concept behind this tutorial is using aireplay-ng replay an ARP packet to generate new unique IVs. In turn, aircrack-ng uses the new unique IVs to crack the WEP key. It is important to understand what an ARP packet is. This "What is an ARP?" section provides the details.

For a start to finish newbie guide, see the Linux Newbie Guide. Although this tutorial does not cover all the steps, it does attempt to provide much more detailed examples of the steps to actually crack a WEP key plus explain the reason and background of each step. For more information on installing aircrck-ng, see Installing Aircrack-ng and for installing drivers see Installing Drivers.

It is recommended that you experiment with your home wireless access point to get familiar with these ideas and techniques. If you do not own a particular access point, please remember to get permission from the owner prior to playing with it.

Assumptions

First, this solution assumes:

You are using drivers patched for injection. Use the injection test to confirm your card can inject prior to proceeding.

You are physically close enough to send and receive access point packets. Remember that just because you can receive packets from the access point does not mean you may will be able to transmit packets to the AP. The wireless card strength is typically less then the AP strength. So you have to be physically close enough for your transmitted packets to reach and be received by the AP. You should confirm that you can communicate with the specific AP by following these instructions.

There is at least one wired or wireless client connected to the network and they are active. The reason is that this tutorial depends on receiving at least one ARP request packet and if there are no active clients then there will never be any ARP request packets.

You are using v0.9 of aircrack-ng. If you use a different version then some of the common options may have to be changed.

Ensure all of the above assumptions are true, otherwise the advice that follows will not work. In the examples below, you will need to change “ath0” to the interface name which is specific to your wireless card.

Equipment used

In this tutorial, here is what was used:

MAC address of PC running aircrack-ng suite: 00:0F:B5:88:AC:82

BSSID (MAC address of access point): 00:14:6C:7E:40:80

ESSID (Wireless network name): teddy

Access point channel: 9

Wireless interface: ath0

You should gather the equivalent information for the network you will be working on. Then just change the values in the examples below to the specific network.

Solution

Solution Overview

To crack the WEP key for an access point, we need to gather lots of initialization vectors (IVs). Normal network traffic does not typically generate these IVs very quickly. Theoretically, if you are patient, you can gather sufficient IVs to crack the WEP key by simply listening to the network traffic and saving them. Since none of us are patient, we use a technique called injection to speed up the process. Injection involves having the access point (AP) resend selected packets over and over very rapidly. This allows us to capture a large number of IVs in a short period of time.

Once we have captured a large number of IVs, we can use them to determine the WEP key.

Here are the basic steps we will be going through:

Start the wireless interface in monitor mode on the specific AP channel

Test the injection capability of the wireless device to the AP

Use aireplay-ng to do a fake authentication with the access point

Start airodump-ng on AP channel with a bssid filter to collect the new unique IVs

Start aireplay-ng in ARP request replay mode to inject packets

Run aircrack-ng to crack key using the IVs collected

Step 1 - Start the wireless interface in monitor mode on AP channel

The purpose of this step is to put your card into what is called monitor mode. Monitor mode is mode whereby your card can listen to every packet in the air. Normally your card will only “hear” packets addressed to you. By hearing every packet, we can later select some for injection. As well, only (there are some rare exceptions) monitor mode allows you to inject packets. (Note: this procedure is different for non-Atheros cards.)

If there are any remaining athX interfaces, then stop each one. When you are finished, run “iwconfig” to ensure there are none left.

Now, enter the following command to start the wireless card on channel 9 in monitor mode:

airmon-ng start wifi0 9

Substitute the channel number that your AP runs on for “9” in the command above. This is important. You must have your wireless card locked to the AP channel for the following steps in this tutorial to work correctly.

Note: In this command we use “wifi0” instead of our wireless interface of “ath0”. This is because the madwifi-ng drivers are being used. For other drivers, use the wireless interface name. Examples: “wlan0” or “rausb0”.

In the response above, you can see that ath0 is in monitor mode, on the 2.452GHz frequency which is channel 9 and the Access Point shows the MAC address of your wireless card. Please note that only the madwifi-ng drivers show the MAC address of your wireless card, the other drivers do not do this. So everything is good. It is important to confirm all this information prior to proceeding, otherwise the following steps will not work properly.

The last line is important. Ideally it should say 100% or a very high percentage. If it is low then you are too far away from the AP or too close. If it is zero then injection is not working and you need to patch your drivers or use different drivers.

Step 4 - Use aireplay-ng to do a fake authentication with the access point

In order for an access point to accept a packet, the source MAC address must already be associated. If the source MAC address you are injecting is not associated then the AP ignores the packet and sends out a “DeAuthentication” packet in cleartext. In this state, no new IVs are created because the AP is ignoring all the injected packets.

The lack of association with the access point is the single biggest reason why injection fails. Remember the golden rule: The MAC you use for injection must be associated with the AP by either using fake authentication or using a MAC from an already-associated client.

Notice the “Got a deauthentication packet” and the continuous retries above. Do not proceed to the next step until you have the fake authentication running correctly.

Troubleshooting Tips

Some access points are configured to only allow selected MAC addresses to associate and connect. If this is the case, you will not be able to successfully do fake authentication unless you know one of the MAC addresses on the allowed list. If you suspect this is the problem, use the following command while trying to do fake authentication. Start another session and…

Notice that the access point (00:14:6c:7e:40:80) is telling the source (00:0F:B5:88:AC:82) you are not associated. Meaning, the AP will not process or accept the injected packets.

If you want to select only the DeAuth packets with tcpdump then you can use: “tcpdump -n -e -s0 -vvv -i ath0 | grep -i DeAuth”. You may need to tweak the phrase “DeAuth” to pick out the exact packets you want.

Step 5 - Start aireplay-ng in ARP request replay mode

The purpose of this step is to start aireplay-ng in a mode which listens for ARP requests then reinjects them back into the network. For an explanation of ARP, see this PC Magazine page or Wikipedia. The reason we select ARP request packets is because the AP will normally rebroadcast them and generate a new IV. Again, this is our objective, to obtain a large number of IVs in a short period of time.

Open another console session and enter:

aireplay-ng -3 -b 00:14:6C:7E:40:80 -h 00:0F:B5:88:AC:82 ath0

It will start listening for ARP requests and when it hears one, aireplay-ng will immediately start to inject it. See the Generating ARPs section for tricks on generating ARPs if your screen says “got 0 ARP requests” after waiting a long time.

Here is what the screen looks like when ARP requests are being injected:

You can confirm that you are injecting by checking your airodump-ng screen. The data packets should be increasing rapidly. The “#/s” should be a decent number. However, decent depends on a large variety of factors. A typical range is 300 to 400 data packets per second. It can as low as a 100/second and as high as a 500/second.

Troubleshooting Tips

If you receive a message similar to “Got a deauth/disassoc packet. Is the source mac associated?”, this means you have lost association with the AP. All your injected packets will be ignored. You must return to the fake authentication step (Step 3) and successfully associate with the AP.

Step 6 - Run aircrack-ng to obtain the WEP key

The purpose of this step is to obtain the WEP key from the IVs gathered in the previous steps.

Note: For learning purposes, you should use a 64 bit WEP key on your AP to speed up the cracking process. If this is the case, then you can include “-n 64” to limit the checking of keys to 64 bits.

Two methods will be shown. It is recommended you try both for learning purposes. By trying both methods, you will see quickly the PTW method successfully determines the WEP key compared to the FMS/Korek method. As a reminder, the PTW method only works successfully with arp request/reply packets. Since this tutorial covers injection of ARP request packets, you can properly use this method. The other requirement is that you capture the full packet with airodump-ng. Meaning, do not use the “--ivs” option.

Start another console session and enter:

aircrack-ng -b 00:14:6C:7E:40:80 output*.cap

Where:

-b 00:14:6C:7E:40:80 selects the one access point we are interested in. This is optional since when we originally captured the data, we applied a filter to only capture data for this one AP.

output*.cap selects all files starting with “output” and ending in “.cap”.

To also use the FMS/Korek method, start another console session and enter:

aircrack-ng -K -b 00:14:6C:7E:40:80 output*.cap

Where:

-K invokes the FMS/Korek method

-b 00:14:6C:7E:40:80 selects the one access point we are interested in. This is optional since when we originally captured the data, we applied a filter to only capture data for this one AP.

output*.cap selects all files starting with “output” and ending in “.cap”.

If you are using 1.0-rc1, add the option “-K” for the FMS/KoreK attack. (1.0-rc1 defaults to PTW.)

You can run this while generating packets. In a short time, the WEP key will be calculated and presented. You will need approximately 250,000 IVs for 64 bit and 1,500,000 IVs for 128 bit keys. If you are using the PTW attack, then you will need about 20,000 packets for 64-bit and 40,000 to 85,000 packets for 128 bit. These are very approximate and there are many variables as to how many IVs you actually need to crack the WEP key.

General Troubleshooting

Generating ARPs

In order for this tutorial to work, you must receive at least one ARP packet. On your home network, here is an easy way to generate an ARP packet. On a wired or wireless PC, ping a non-existent IP on your home LAN. A wired PC means a PC connected to your LAN via an ethernet cable. Lets say your home LAN address space is 192.168.1.1 through 192.168.1.254. Pick an IP between 1 and 254 which is not assigned to a network device. For example, if the IP 192.168.1.213 is not being used then “ping 192.168.1.213”. This will cause an ARP to be broadcast via your wireless access point and in turn, this will kick off the reinjection of packets by aireplay-ng.